The present invention relates to free-flowing, thermoplastically processible and post-crosslinkable polyurethane powders, their production and their use for the production of crosslinked sheets by processing above the melting point via the powder slush molding process.
The invention further relates to expandable polyurethane powders containing blowing agents and to their use for the production of foamed polyurethane moldings.
Powders play an important part in many fields of plastics production and processing. Due to their high degree of dispersion and the flowability associated therewith, they are used, e.g., for the production of adhesives, coatings, thin films and sheets.
The preparation of cellular (or foamed) moldings from microcellular polyurethane elastomers or polyurethane foams is known and has been described in numerous patent and literature publications (see e.g., Kunststoff-Handbuch, volume 7, "Polyurethane", 1st Edition, 1966, 2nd Edition, 1983, Carl Hanser Verlag, Munich, Vienna). ("Polyurethane" will hereinafter be abbreviated to "PU") These moldings are normally produced by introducing pourable or fluid reaction mixtures of organic polyisocyanates and compounds having at least two reactive hydrogen atoms together with liquid blowing agents into molds which are optionally tempered, and then foaming up and curing the mixtures in these molds.
It is known that polyurethane moldings having a compact, substantially non-porous outer skin or surface and a cellular core (PU integral foams) are produced by introducing a larger quantity of foamable reaction mixture into the mold than would be required for completely filling the cavity of the mold with the foamed up mixture without the application of pressure. Foaming is thus carried out inside a closed mold under conditions of compression.
Thin panels or films of PU foam produced in large PU foam blocks or by bonding PU foam waste under pressure are used for numerous applications, e.g. for soundproofing in the engine compartment of motor vehicles. It would be desirable to be able to produce the foam parts in the desired form in as few working steps as possible but the foaming and curing process of the usual liquid reactive systems for such small foam parts is difficult to carry out.
The use of decorative plastic sheets in motor vehicle passenger compartments is known. [R. Pfriender, Kunststoffe, 76 (1986), 10, p. 960 ff]. According to the prior art in this connection, PVC/ABS sheets formed by the thermoform process are usually used and are then back-foamed. Polyurethane sheets produced from liquid systems can be processed by the IMC (in-mold coating) process. The production of components, however, is very expensive and difficult to control (M. Wachsmann, Kunststoffberater 10/1987, pp. 27-28).
On the other hand, the powder slush molding process permits the production of sheets that are considerably finer-structured and undercut and accordingly of higher quality. Here, according to the prior art, PVC powder is used which is gelled at about 250.degree. C. by rotating in the heated mold. After cooling the mold, the sheet can be withdrawn. Here again, however, it is necessary to transfer the resulting film into another mold. In other words, back-foaming must be carried out in a second mold.
For the PVC sheets, which are usually back-foamed with polyurethane foams, there is a mutually negative effect between the PVC sheet and of the PU foam. Catalysts or stabilizers can diffuse out of the PU foam into the PVC skin. There is simultaneously a migration of e.g. plasticizers in the reverse direction. These processes may cause mechanical damage or discoloration to the molded products, e.g. the products may become brittle. These damaging interactions can be avoided by using PU powders for producing the covering layers and back-forming them with PU foam. This has the added advantage of producing a uniform synthetic resin bond which can more easily be worked up again. It would be advantageous to provide a simpler process than that of the state of the art so that the skin produced could be directly back-foamed in the same mold in which it was produced.
From the viewpoint of reworkability, moreover, composite systems of a uniform type of plastic are desirable. Accordingly there is a strong interest from the automobile industry in uniform materials in which the aforementioned negative interactions between skin and back-foaming do not occur and the possibility of simpler recycling exists. The use of PU cover sheets that are back-foamed with PU therefore suggests itself.
It is known to granulate and grind thermoplastic polyurethanes (TPU) that are obtained by extrusion or ribbon processes to obtain PU powders [German Offenlegungsschriften 3,916,874 (believed to correspond to Canadian patent 2,017,354) and 4,006,648]. The resultant granulate can be processed by sintering to sheets. This procedure is disadvantageous due to the expensive grinding process required. Due to the toughness of the materials used, the system must also be strongly cooled, e.g. with liquid nitrogen, during grinding. In addition, expensive devices for avoiding dust are required. Moreover the irregular shape of the material being ground can cause problems with the flowability of the powder.
The possibility of producing cellular PU moldings, preferably PU sheets, from a mixture of the resulting TPU powders with a blowing agent which is solid at 23.degree. C. by sintering the mixture has also been described. These PU powder mixtures containing blowing agent may be used, for example, in the above-mentioned powder slush process for back-foaming the previously produced skin without having to change the mold (see, e.g., German Auslegeschriften 4,006,648).
This procedure, i.e. the use of TPU produced by extrusion or laminator processes, is disadvantageous due to the subsequent grinding process necessary for producing the powder. Due to the toughness of the materials used, grinding must be accompanied by vigorous cooling, e.g. with liquid nitrogen. In addition, expensive apparatus are required for preventing the formation of dust. Further, the irregular form of the grinding stock may cause problems in the flowability of the powder. Due to the high temperatures of the extrusion or laminator process, preparation of the TPU powder mixture containing blowing agent must be carried out by subsequently mixing the powder with the blowing agent which is solid at 23.degree. C. and also pulverulent, so that an additional, expensive processing step is required.
The use of thermoplastic PU systems permits the sintering of powders with flow of the melt to a homogeneous skin. But with pure thermoplastic (linear) systems, the low-viscosity TPU melt flows downwards or away on vertical mold surfaces or at overheated points of the mold, so that the thickness of the skin can become non-uniform. Holes can even appear in the sheet.
Thermoplastic processing requires a system that initially is substantially linear. Crosslinking occurring during processing is disadvantageous due to the resulting increase of viscosity.
It is known to react isocyanates with monofunctional reactants ("blocking agents") such as, e.g. oximes, caprolactam or phenol derivatives to form thermolabile adducts. These "capped" or "blocked" isocyanates can be used to prepare systems which are thermoplastic up to the breakdown temperature of the adducts and which crosslink above this temperature.
Furthermore, the use of uretdiones or polyisocyanates containing uretdione groups in one-component PUR reactive systems is known. The polyisocyanates containing uretdione groups are obtained by bringing two isocyanates together (i.e., "dimerizing") in presence of special catalysts. The uretdione groups split at elevated temperature and react further, e.g., in presence of hydroxy groups with crosslinking. This occurs without release of any "capping" or "blocking" agent. The production of such one-component PUR systems containing capped isocyanates by the conventional extrusion or ribbon process in the melt, as described e.g. in the German Offenlegungsschrift 3,916,874, is basically problematic, since the cleavage temperature can be reached or exceeded and the system crosslinks even during its production. In addition, the product must then be ground to powder in a way that is costly and difficult.
On the other hand, the direct production of PU powders by polyaddition in emulsion is a process that provides free-flowing powders directly and at relatively moderate reaction temperatures (see, e.g., U.S. Pat. No. 4,985,490 and European patent 394,789). The direct production of pulverulent thermoplastic polyurethanes by reaction of polyester or polyether polyols and low-molecular diols with polyisocyanates in an inert solvent in presence of emulsifiers is known.
It is also known is to directly produce pulverulent, completely blocked isocyanates by reaction of polyisocyanates with NCO-reactive compounds (German Offenlegungsschrift 2,536,976, believed to correspond to U.S. Pat. No. 3,963,710). It is also known to produce two-component thermally activable powders (a powder mixture of two powders of different chemical composition) in which one kind of particle has isocyanate groups blocked with NCO-reactive compounds (German Offenlegungsschrift 2,556,945, believed to correspond to U.S. Pat. No. 3,933,759). The complete blocking of the NCO groups in the powder leads to too rapid a build-up of viscosity during the crosslinking. The production of two-component powders via a mixing process leads to inhomogeneities which have an adverse effect on the reaction product.
The possibility of using blocked isocyanates in the production of PU powders in a continuous process has been suggested (European patent 394,789, believed to correspond to U.S. Pat. No. 4,940,750). Specific advantages relating to the use of the blocked NCO groups, i.e. the advantageous melt behavior, in the powder slush molding process, were not suggested or recognized.
It was an object of the present invention to develop a PU powder suitable for use in the slush molding process, that was free-flowing, that at first melts like a thermoplastic and then advantageously progressively crosslinks, which does not have the risk of pre-crosslinking under production conditions and furthermore avoids any solid mixing processes.
It was a further object of the invention to provide suitable pulverulent, fusible polyurethane systems not requiring a difficult grinding process or subsequent mixing with blowing agents for the production of cellular PU moldings in relatively thin layers or the back-foaming of compact films.